The present invention relates to a method for producing a support for a planographic printing plate, and particularly relates to a method for producing an aluminum support having excellent electrolytic graining properties.
An aluminum plate (including aluminum alloy) is often used as a printing plate, particularly, a printing plate for use as an offset printing plate. In using an aluminum plate used as an offset printing plate support, it is generally necessary that the aluminum plate have a good adhesive property to a photosensitive layer and good water retentivity. For this purposes, the aluminum plate must be roughened to provide the aluminum plate with a uniform and finely grained surface. Because this roughening treatment has a remarkable influence on printing characteristics and durability during offset printing, the effect of the roughening treatment is an important factor in the production of the plate material.
An AC electrolytic graining method is generally employed for roughening an aluminum support for a printing plate. For the waveform of the current used in such a method, an ordinary sinusoidal wave form alternating current or a special waveform alternating current such as a square waveform alternating current, etc., can be employed. Graining of an aluminum plate is performed using such an alternating current with a suitable electrode such as a graphite electrode as a counter electrode.
The graining can generally be completed in one treatment, but in such a case the depth of the pits obtained by the graining is generally small, so that the resulting aluminum support is inferior in durability. Therefore, various methods have been proposed to obtain a suitable aluminum plate for use as a support for a printing plate having a grained surface in which pits having depths larger than their diameter are formed evenly. Examples of such methods are disclosed in Japanese Patent Unexamined Publication No. Sho. 53-67507, Japanese Patent Unexamined Publication No. Sho. 54-65607, Japanese Patent Unexamined Publication No. Sho. 55-25381, and Japanese Patent Unexamined Publication No. Sho. 56-29699, etc. Further, a method using a combination of an AC electrolytic etching and mechanical graining treatments is disclosed, for example, in Japanese Patent Unexamined Publication No. Sho. 55-142695.
A known method for producing an aluminum support includes steps of casting a slab (with a thickness of 400 to 600 mm, a width of 1000 to 2000 mm, and a length of 2000 to 6000 mm) by melting and holding an ingot of aluminum, applying a surface cutting to remove a thin portion (about 3 to 10 mm) from the surface of the slab to thereby remove the impurity-structure surface portion, evenly heating the slab in a furnace at a temperature of 480.degree. to 540.degree. C. for 6 to 12 hours in order to remove stress inside of the slab and equalize the surface portions of the slab, and then hot-rolling the slab at a temperature of 480.degree. to 540.degree. C. After the slab is hot-rolled to a thickness of 5 to 40 mm, the slab is cold-rolled to a predetermined thickness at room temperature. Then, to homogenize the surfaces and to make the plate excellent in flatness, annealing is carried out to thereby homogenize the rolled structure and the like. Then, cold rolling is carried out to obtain a predetermined thickness, and finally correction is carried out. The aluminum support thus produced is used as a support for a planographic printing plate.
An electrolytic graining treatment is apt to be affected by the characteristics and composition of the aluminum support subjected to the treatment. That is, in the production of an aluminum support through the steps of melting/holding, casting, surface cutting and soaking, there can arise variations of the components of the metal alloy in the surface layer, even in the case where not only heating and cooling are alternately carried out, but also surface cutting is employed, that is, a step of cutting away the surface layer is carried out. This causes a lowering of the yield rate of the aluminum support for a planographic printing plate.
To reduce variations in the quality of the material of the aluminum support so as to improve the yield rate in the electrolytic graining treatment and to thereby produce a planographic printing plate excellent both in quality and in yield, there has been proposed a method for producing a support for a planographic printing plate including steps of forming a hot-rolled thin-plate coil by continuously carrying out casting from molten aluminum and hot rolling, applying cold rolling, a heat treatment and applying correction to the coil to thereby obtain an aluminum support, and then graining the aluminum support (see U.S. Pat. No. 5,078,805 which corresponds to Japanese Patent Unexamined Publication No. Hei. 3-79798).
In such a method, however, there can still arise variations in the electrolytic graining treatment of the plate. In addition, stripe irregularities sometimes occur in the grained surface so that the external appearance of the plate is sometimes poor.